#6 renaming / deciding on language

Currently most of the function names, variables etc. have english names, although some column names of the output tibbles are in german.

We should decide on one language.

if we decide to do the presentation and documentation in german, maybe in this case german might be easier, I for my part prefer not to write german documentation with english variable names. However, we also could just do everything in english.

Refactoring done, all variable names are in English now.